Wire bonding systems and related methods

ABSTRACT

A wire bond system. Implementations may include: a bond wire including copper (Cu), a bond pad including aluminum (Al) and a sacrificial anode electrically coupled with the bond pad, where the sacrificial anode includes one or more elements having a standard electrode potential below a standard electrode potential of Al.

CROSS REFERENCE TO RELATED APPLICATIONS

This document claims the benefit of the filing date of U.S. ProvisionalPatent Application Ser. No. 62/149,455 (the '455 Provisional), entitled“Bond Wires and Methods” to Qin et al. which was filed on Apr. 17, 2015and also claims the benefit of the filing date of U.S. ProvisionalPatent Application Ser. No. 62/256,130 (the '130 Provisional) entitled“Wire Bond Structures and Methods” to Qin et al. which was filed on Nov.17, 2015, the disclosures of each of which are hereby incorporatedentirely herein by reference.

BACKGROUND 1. Technical Field

Aspects of this document relate generally to structures and systems usedfor wire bonding.

2. Background

Wire bonding involves thermally, mechanically and electricallyconnecting a wire with a bonding pad. In various ball wire bondingsystems, ultrasonic or other energy is used to melt the wire to form afree ball that is then pressed down on the bond pad. The process resultsin the mixing of the materials of the wire and the bond pad, forming awire bond.

SUMMARY

Implementations of wire bond systems may include: a bond wire includingcopper (Cu), a bond pad including aluminum (Al) and a sacrificial anodeelectrically coupled with the bond pad, where the sacrificial anodeincludes one or more elements having a standard electrode potentialbelow a standard electrode potential of Al.

Implementations of wire bond systems may include one, all, or any of thefollowing:

The one or more elements may be one of magnesium (Mg), hafnium (Hf),beryllium (Be) or any combination thereof.

Implementation of wire bond systems may include a bond wire includingCu, a bond pad including Al, and a sacrificial anode physically andelectrically coupled over at least a portion of bond pad where thesacrificial anode includes one or more elements having a standardelectrode potential below a standard electrode potential of Al.

Implementations of wire bond systems may include one, all, or any of thefollowing:

The one or more elements may be one of Mg, Hf, Be, or any combinationthereof.

Implementations of a wire bond system may include a bond wire includingCu and a bond pad coupled to the bond wire where the bond pad includes amaterial including Al and one or more elements having a standardelectrode potential between a standard electrode potential of Cu and astandard electrode potential of Al.

Implementations of wire bond systems may include one, all, or any of thefollowing:

The one or more elements may include tungsten (W).

The one or more elements may include zinc (Zn).

The one or more elements may include chromium (Cr).

The one or more elements may include tin (Sn).

The one or more elements may include iron (Fe).

The one or more elements may be selected from the group consisting ofmolybdenum (Mo), cadmium (Cd), cobalt (Co), nickel (Ni), Cu, or anycombination thereof.

The bond wire may include a coating including a metal selected from thegroup consisting of gold (Au), silver (Ag), palladium (Pd), nickel (Ni),or any combination thereof.

The bond wire may further include Ni.

Implementations of a wire bond system may include a bond wire includingCu, a bond pad coupled to the bond wire, the bond pad including Al, anda layer coupled to the bond pad between the bond wire and the bond pad,where the layer includes one or more elements having a standardelectrode potential between a standard electrode potential of Al and astandard electrode potential of Cu.

Implementations of wire bond system may include one, all, or any of thefollowing:

The one or more elements may include W.

The one or more elements may include Zn.

The one or more elements may include Cr.

The one or more elements may include Sn.

The one or more elements may be selected from the group consisting ofMo, Cd, Co, Ni, Fe, Cu, or any combination thereof.

The bond pad may further include one or more elements having a standardelectrode potential between a standard electrode potential of Cu and astandard electrode potential of Al.

The foregoing and other aspects, features, and advantages will beapparent to those artisans of ordinary skill in the art from theDESCRIPTION and DRAWINGS, and from the CLAIMS.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations will hereinafter be described in conjunction with theappended drawings, where like designations denote like elements, and:

FIG. 1 is a perspective view of a wire bonded to a pad;

FIG. 2 is a cross sectional view of a wire bonded to a padimplementation;

FIG. 3 is a cross sectional view of a wire bonded to a pad showing asacrificial anode implementation coupled with the pad;

FIG. 4 is a cross sectional view of a wire bonded to a pad illustratinga sacrificial anode implementation coupled over the pad around the wire;

FIG. 5 is a cross sectional view of a wire bonded to a pad illustratingan implementation of a pad comprising an implementation of a layerformed between the wire and the pad;

FIG. 6 is a cross sectional view of a wire bonded to a pad illustratinganother implementation of a layer formed between the wire and the pad.

DESCRIPTION

This disclosure, its aspects and implementations, are not limited to thespecific components, assembly procedures or method elements disclosedherein. Many additional components, assembly procedures and/or methodelements known in the art consistent with the intended wire, pad, andwire bonding systems will become apparent for use with particularimplementations from this disclosure. Accordingly, for example, althoughparticular implementations are disclosed, such implementations andimplementing components may comprise any shape, size, style, type,model, version, measurement, concentration, material, quantity, methodelement, step, and/or the like as is known in the art for such wires,pads, and wire bonding systems, and implementing components and methods,consistent with the intended operation and methods.

Referring to FIG. 1, a perspective view of a bond wire (wire) 2 bondedto a bond pad (pad) 4 is illustrated. In this implementations, the wire2 has been bonded to the pad 4 using a ball bonding process. Asillustrated, during the bonding process, the wire 2 is melted and theresulting ball is then connected to the pad. A combination of ultrasonicenergy with heat as a byproduct, or ultrasonic energy with external heatsource softens the ball and causes the softened material to flow underload when pressed down on the pad 4, causing the two materials tomix/stick to one another. As can be seen, during this process, dependingon the hardness of the ball on the wire, a certain amount of pad metaldisplacement (PMD) or aluminum splash 6 can be observed around the edgesof the wire. Because of the temperatures/energies involved duringvarious wire bonding processes, whether ball bonding or wedge bonding,various mixtures of the materials of the wire and the pad are observedafter the bonding process is completed. Where the wire 2 and the pad 4are made of metallic materials, the mixtures are alloys which includevarious intermetallic compounds (IMCs) of the metals of the wire 2 andthe pad 4. FIG. 2 illustrates a cross section of a wire bond like thatillustrated in FIG. 1, showing the wire 2, the pad 4, and an alloy stack8. While in FIG. 2, the alloy stack 8 is illustrated as extendingupwardly toward the wire 2, in various implementations, the alloy stack8 may also extend downwardly into the pad 4. In various implementations,there may be no detectable alloy stack 8 immediately following bonding,but the alloy stack 8 becomes easily detected after operation of thesemiconductor package and/or testing.

Where the wire 2 contains copper (Cu) and the pad contains aluminum(Al), the potential for corrosion to take place exists following thebonding process. This is because the standard Cu/Cu²⁺ electrode has astandard electrode potential of +0.342 eV and the standard Cu/Cu¹⁺electrode has a standard electrode potential of +0.521 eV while thestandard Al/Al³⁺ electrode has a standard electrode potential of −1.66eV. In the various system implementations disclosed in this document,either of the Cu standard electrode potentials can be used as areference. Because the potentials are different, in the presence of ahumid environment, it is possible for galvanic corrosion to take place.Without being bound by any theory, it is also believed that any halogenscontained in mold compounds surrounding the wire bond can furtheraccelerate the corrosion process. Because of the corrosion, the IMCsbecome oxidized and cracks can spread between the wire and the pad,resulting in electrical separation of the two. Because extensive amountsof time may be required before the corrosion creates this result, thecorrosion process creates a reliability concern that cannot be detectedin the initial test process of a semiconductor package.

The '455 Provisional previously incorporated by reference, and thearticle by Qin et al., entitled “Corrosion Mechanisms of Cu Bond Wireson AlSi Pads,” Conference Proceedings from the 41^(st) InternationalSymposium for Testing and Failure Analysis, p. 423-428 (November 2015),(Qin et al.) the disclosure of which is hereby incorporated entirelyherein by reference, each discloses in detail proposed mechanisms forthe corrosion process between Cu and Al following wire bonding observedfollowing Highly Accelerated Stress Testing (HAST) testing. Withoutbeing bound by any particular theory, it appears that where the wirescontain Cu and the pad contains an alloy of AlSi, various Cu-rich IMCsformed which then, during the testing, subsequently corrode.

In this document, various wire, pad, and wire bonding systemimplementations that are designed to minimize/mitigate the potential forcorrosion occurring between a Cu-containing wire and an Al-containingpad are disclosed. A wide variety of Cu-containing bond wires may beused in various implementations. Some of these may include, bynon-limiting example, 1N, 2N, 3N, 4N, 5N, and 6N copper wire; coatedcopper wire; and alloyed copper wire. By non-limiting example, any ofthe alloyed copper wire versions disclosed in the following referencesmay be used in various implementations: U.S. Patent ApplicationPublication No. 20130142567 to Sarangapani et al., entitled “Doped 4Ncopper wires for bonding in microelectronics devices,” filed Nov. 29,2012; U.S. Patent Application Publication No. 20130142568 to Sarangapaniet al., entitled “3N copper wires with trace additions for bonding inmicroelectronics devices,” filed Nov. 30, 2012; U.S. Patent ApplicationPublication No. 20130140068 to Sarangapani et al., entitled “Secondaryalloyed 1N copper wires for bonding in microelectronics devices,” filedNov. 29, 2012; and U.S. Patent Application Publication No. 20130140084to Sarangapani et al., entitled “Alloyed 2N copper wires for bonding inmicroelectronics devices,” filed Nov. 30, 2012; the disclosures of eachof which are hereby incorporated entirely herein by reference. Bynon-limiting example, implementations of coated copper wire that couldbe used in various implementations disclosed herein may include wirescoated with tin (Sn), silver (Ag), nickel (Ni), palladium (Pd), gold(Au), an organic (carbon containing) material, a ceramic material, orany combination thereof, including any of the coating types disclosed inU.S. Patent Application Publication No. 20150311173 to Carpenter et al,entitled “Structures and methods for reducing corrosion in wire bonds,”filed Apr. 25, 2014, the disclosure of which is hereby incorporatedentirely herein by reference.

In a first implementation, a wire bond system includes a bond wire thatincludes Cu and a bond pad that includes Al. Referring to FIG. 3, animplementation of such a system with a wire 10 and pad 12 isillustrated. Also illustrated is a sacrificial anode 14, which iselectrically coupled with the pad 12 by being placed directly onto thepad 12. In various implementations, however, the sacrificial anode 14may not be placed in direct physical contact with the pad 12, but may beplaced electrical contact with the pad 12 while still being in contactwith the electrolyte(s) involved in a corrosion reaction. In this way,in various implementations, more than one pad may be electricallycoupled with a single sacrificial anode.

The sacrificial anode 14 may be placed in electrical contact eitherprior to or after the wire bond has been formed, depending on theimplementation.

The sacrificial anode may include one or more elements that have astandard electrode potential below a standard electrode potential of Al.By non-limiting example, the one or more elements may be magnesium (Mg)or an alloy of Mg and Al. In particular implementations, the one or moreelements may be Be or an alloy of Be and Al. In other implementationsHf, uranium (U), Be, sodium (Na), calcium (Ca), potassium (K), lithium(Li) Mg, or any combination of thereof could be used. In variousimplementations, the sacrificial anode may include various combinationsof materials and/or alloys, including composites having the desiredstandard electrode potentials. Because the sacrificial anode has astandard electrode potential below that of Al, Al becomes the cathoderelative to the sacrificial anode, which significantly slows down thecorrosion at the wire/pad interface.

Referring to FIG. 4, a second implementation of a wire bond system 16 isillustrated. As illustrated, a wire 18 is coupled to a pad 20, the wireincluding Cu and the bond pad including Al. A sacrificial anode 22 isalso included which is physically and electrically connected to the bondpad by being coupled over it. In this implementation, the sacrificialanode 22 is created through deposition/patterning/etching steps duringprocessing of the semiconductor die itself, and is covered by thepassivation layer(s) 24 of the semiconductor die. In various methodimplementations, the sacrificial anode 22 material may be deposited overthe pad 20 via physical vapor deposition (PVD), chemical vapordeposition (CVD), metalorganic chemical vapor deposition (MOCVD),electroplating, electroless plating, and any other technique for forminga material over the pad material. When the pad 20 is opened, thepassivation layer 22 is etched away and the sacrificial anode materialmay be either etched as a separate step or simultaneously during thepassivation layer etch. Alternatively, in other method implementations,the sacrificial anode material may be patterned and etched to expose thepad 20 prior to the deposition and subsequent etching of the passivationmaterial.

Similar to the first system implementation, the material of thesacrificial anode includes one or more elements that have a standardelectrode potential below a standard electrode potential of Al, whichmay be any previously disclosed, including Mg, Be, Hf, U, Na, Ca, K, Li,and any combination thereof. In the first and second systemimplementations, any of the Cu-containing wire implementationspreviously disclosed in this document may also be used.

Referring to FIG. 2 for reference, the structure of a third systemimplementation can be illustrated. In such an implementation, the wire26 contains Cu and the bond pad 28 is coupled to the bond wire. The pad28 includes a material containing Al and one or more elements that havea standard electrode potential between a standard electrode potential ofCu and a standard electrode potential of Al. In various implementations,the material may be a metal alloy, a composite, or other material thatincludes Al. Because the elements and/or other materials included in thebond pad have a standard electrode potential between those of Cu and Al,the overall bond pad itself will have a higher electrode potential thanAl alone. Because of this, the rate at which any theoretical corrosionreaction could take place is lowered, because the potential differencehas been lowered. Lowering the theoretical reaction rate may have theeffect of mitigating the corrosion reliability risk over the lifetime ofthe semiconductor package.

A wide variety of elements and combination of elements could be used invarious system implementations. For example, the element used inaddition to Al could be tungsten (W), zinc (Zn), chromium (Cr), Sn,silicon (Si), cadmium, (Cd), molybdenum (Mo), cobalt (Co), Cu, or anycombination of these. In various implementations, other elements orcompounds could also be used which have a standard electrode potentialbetween the standard electrode potentials of Cu and Al. Also, variousimplementations of the third system may include any of the bond wireimplementations, including wire alloys and coatings disclosed in thisdocument. In these implementations, any combination of the bond wireimplementations may be used in combination with the bond pad materialimplementations disclosed herein.

Referring to FIG. 5, an implementation of a fourth system implementation30 is illustrated. As illustrated, the system 30 includes a wire 32 anda bond pad 34. The wire 32 includes Cu and bond pad 34 includes Al, andthey may be made of any wire or pad material disclosed in this document.A layer (coating) 36 is present that is coupled over the bond pad 34 andcontains a material that includes one or more elements that have astandard electrode potential between a standard electrode potential ofAl and a standard electrode potential of Cu. This layer 36 may be asingle layer of homogenous material, whether an alloy or composite invarious implementations. In others, it may be formed of various layersof different material types that have been deposited or formed one onthe other. In some implementations, the layer 36 may extend entirelyacross the bond pad 34, but in others, the layer 36 may be located atthe point where the wire 32 is to couple to the pad 34.

In various implementations, depending on the materials selected for thelayer 36, its thickness, etc., the hardness mismatch that exists betweenCu and Al can also be reduced, which can reduce the likelihood of paddamage occurring during wire bonding. Also, since materials like Zn andCd are harder than Al, they may increase the contact area with theAl-containing pad and reduce the splash of the bond, similar to thebenefits seen with Au wire bonding. The thickness of the layer may beabout 30 angstroms to about 200 angstroms thick in variousimplementations. In various implementations, electroless depositionprocesses such as zincation may be used to avoid having to usepatterning and etching processes.

Referring to FIG. 6, another implementation 38 of a fourth systemimplementation is illustrated. In this implementation, a layer 44 iscoupled over the pad 42 between the wire 40 and the pad 42. However,during the bonding process (and subsequent use), the material of thelayer 44 may intermix/diffuse/react with the material of the pad 42and/or the material of the wire 40, forming an IMC layer 46, if thematerials are metals, or another solid mixture, solution, and/or alloy.The IMC layer 46 may comprise one or more elements with a standardelectrode potential between those of Cu and Al, or the resultingcompounds formed in the IMC layer may have a standard electrodepotential between Cu and Al. In this way, a barrier to forming Cu-richIMC compounds may be formed that may likely reduce the rate of thecorrosion reaction, or a mixture/solid solution formed that enhancesadhesion between the wire 40 and the pad 42, preventing cracking andvoiding if corrosion were to take place.

Any of the elements and combinations of elements disclosed for use withthe bond pad of the third system implementation may be used in the layerimplementations 36, 44, including, by non-limiting example, W, Zn, Cr,Sn, Mo, Cd, Co, Cu, Al or any combination of these. In particularimplementations, the layer may be a layer of Zn. Also, other materialsand compounds with the desired standard electrode potential values couldbe used in various implementations. Furthermore, the bond pad 34 itselfmay include Al and any of the one or more elements having a standardelectrode potential between the standard electrode potential of Cu andthe standard electrode potential of Al disclosed in this document. Thiscombination may further, in various implementations, be able to creatematerials with different standard electrode potentials capable ofreducing the rates of corrosion reactions.

In some system implementations, the pad may include Al exclusively orAlSi and the wire may be an alloy like any of those disclosed in thisdocument. In various implementations, the wire may include a CuNi alloy.

EXAMPLE 1

A set of 43 bipolar junction transistors each including a base pad andan emitter pad were used as part of an experiment to measure the wirebonding characteristics of various combinations of a Cu wire and padmaterial combinations. A pad that was an alloy of AlSi was used as thecontrol and the following pad material types were used: AlCuW alloy,AlCu, and AlCuSi. The AlCu and AlCuSi alloys were deposited using aphysical vapor deposition tool marketed under the tradename ENDURA byApplied Materials of Santa Clara, Calif. The AlCuW layer was depositedusing a physical vapor deposition process using an AlCuW target. All ofthe various parts were wirebonded and evaluated post wirebonding.Analysis of all of the pads for the various materials including thecontrol showed that all the parts met the specification of >75% for thepercentage of the bonded area on the pad.

Table 1 summarizes the result of pad metal displacement-remaining(PMDr), all of the pads met the specification value of deformationbetween 20% to 80%.

TABLE 1 Pad AlCuW AlCu AlCuSi AlSi Base 68% 59% 65% 57% Emitter 70% 67%61% 54%

On the average, AlCuW had the best PMDr for both the base and theemitter pads. AlCuSi has better PMDr than AlSi, and AlCu is better thanAlCuSi for the Emitter pad. Generally, the trend in progressively highervalues of PMDr of AlCuW>AlCu>AlCuSi (except for base pad)>AlSi wasobserved.

HAST testing of the parts indicated that, in contrast with the partswith AlSi pads, the AlCuW, AlCu, and AlCuSi pads all passed the test.

Given that the AlCuW alloy pad contains an element that has an effectivestandard electrode potential between Cu and Al, the rate of thecorrosion reaction may be reduced. In addition, W is believed torestrict segregation of Cu in the pad and react with the Al materialwhich may further serve to prevent formation of Cu-rich IMC compoundsthat are vulnerable to corrosion, as discussed in the '455 provisionaland in the paper by Qin et al.

In places where the description above refers to particularimplementations of wire, pad, and wire bonding systems and implementingcomponents, sub-components, methods and sub-methods, it should bereadily apparent that a number of modifications may be made withoutdeparting from the spirit thereof and that these implementations,implementing components, sub-components, methods and sub-methods may beapplied to other wire, pad, and wire bonding systems.

What is claimed is:
 1. A wire bond system comprising: a bond wirecomprising Cu; a bond pad comprising an aluminum alloy; and asacrificial anode electrically coupled with the bond pad; wherein thesacrificial anode does not physically contact the bond pad.
 2. Thesystem of claim 1, wherein the sacrificial anode comprises one of Mg,Hf, Be, or any combination thereof.
 3. A wire bond system comprising: abond wire comprising Cu; a bond pad comprising an aluminum alloy; asacrificial anode physically and electrically coupled over at least aportion of the bond pad; and one of an alloy stack or an intermetalliclayer formed through bonding the bond wire through the sacrificial anodewith the bond pad.
 4. The system of claim 3, wherein the sacrificialanode comprises one of Mg, Hf, Be, or any combination thereof.
 5. A wirebond system comprising: a bond wire comprising Cu; a bond pad coupled tothe bond wire, the bond pad comprising an aluminum alloy; a layerforming a sacrificial anode coupled to the bond pad between the bondwire and the bond pad; and one of an alloy stack or an intermetalliclayer formed through bonding the bond wire through the layer with thebond pad.
 6. The system of claim 5, wherein the layer comprises W. 7.The system of claim 5, wherein the layer comprises Zn.
 8. The system ofclaim 5, wherein the layer comprises Cr.
 9. The system of claim 5,wherein the layer comprises Sn.
 10. The system of claim 5, wherein thelayer comprises one or more elements selected from the group consistingof Mo, Cd, Co, Ni, Fe, Cu, and any combination thereof.